In this lesson we're going to go over some considerations for using a LiDAR-derived DEM. We'll be introducing concepts such as hydro-modification of a DEM and some pre-processing that you should do on your DEM before conducting flow modeling across the landscape.

When you download your DEM, it's very important to look at the metadata to see what, if any, pre-processing has been performed on that DEM.

The example below shows hydro-flattening, which is generally performed by the vendor who is collecting the LiDAR. Hydro-flattening is what is done to flatten elevations within water bodies and wide rivers. This is done because LiDAR is not collected over water. The pulse that is sent out is actually absorbed by the water instead of returned back to the sensor like it is over land.

Since there is no data, the water surface is interpolated using the bank elevation, which creates ridges as shown in the left-side image, and these ridges can act as dams which will impede flow. In these lakes or water bodies and wide rivers, the ridges have been flattened, smoothing the surface. A constant downhill gradient is also created, which is important in water bodies like rivers.

A second process that can be done to your DEM is hydro-conditioning, which is where obstructions to flow are cut through, allowing the water to continue moving across and down the watershed.

Seen in this example, we have roadways that would have naturally impeded the channel flow based on the LiDAR surface. We can view it as a dam, but now it's been cut, which allows the water to flow through. This is not likely to have been done by the vendor since it's a very detailed and time-consuming process. It's extremely necessary, however, to produce an accurate flow model and therefore a more reasonable watershed plan.

To give an example why hydro-conditioning is so important, notice in the image below where a cut line has not been made through this road. This will result in an interrupted flow path. Everything above the road moves along as expected, however, when it hits the roadway, the software treats it as a dam. The flow begins to back up behind the roadway creating a ponding effect until it reaches a height that it can spill over the roadway. When that happens, the flow path is no longer falling into the correct channel resulting in a skating rink effect visible in the lower image. This demonstrates how important it is to have the local knowledge to be able to make cuts where you know they exist.

Hydro enforcement.JPG

The ACPF has been built to assist you in this process of hydro-conditioning. There are tools that will guide you step-by-step through this process.

The three tools involved in the hydro-conditioning process include:

1. Impeded Flowpaths

2. Visualize Flowpaths

3. Manual Cutter and Dam Builder

These three tools will be run iteratively to create an accurately hydro-conditioned DEM. It's important that you take your time and are diligent throughout this process. If you don't take your time or you skip steps, you're going to lack an accurate flow network.

For more information please view the video below (runtime 5:05):